The inadequacy of commercial-off-the-shelf capacitors in meeting the challenges of high voltage, pulse-power driven applications makes it imperative to design next generation dielectric materials for the development of high energy density storage devices. These are suitable for a variety of energy delivery systems such as high power microwave sources, lasers, particle beam accelerators, space power conditioning in satellites and spacecraft as well as oil drilling and mining operations. Such energy sources should also be compact, especially for mobile applications, underscoring the significance of weight savings for high-energy capacitive storage.
The volumetric energy density of an electrostatic capacitor is expressed in the following equation:Dv=∈∈0E2/2 J/ccwhere ∈ is the relative dielectric constant of the medium, ∈0 is the permittivity of free space (8.85×10−14 F/cm) and E is the applied field in V/cm.
The gravimetric energy density, expressed as J/g, is obtained as follows:Dg=∈∈0E2/2ρwhere ρ is the material density. Enhanced energy density would require increased dielectric constant or the maximum voltage applied prior to breakdown or both, while keeping the density to a minimum. Since energy density varies as the square of the applied electric field, there is a greater payoff in attaining higher breakdown voltage (BDV) for the dielectric study.
Polymer dielectrics are the preferred materials of choice for such high voltage, pulse power capacitor applications because of their potential for high breakdown strengths, low dissipation factors and good dielectric stability despite having inherently lower dielectric constants relative to ceramic capacitors. Among the metallized thin film capacitors known commercially, the biaxially oriented polypropylene (BOPP) is known to exhibit the highest breakdown strength (˜650 V/μ, 16 kV/mil.) and a desirably low dielectric loss factor or dissipation factor (10−4 at 1 kHz) but its drawbacks are a very low dielectric constant (2.2) and a low service temperature (˜105° C.).
The commercial polyester dielectric PET, also used in capacitors, has a dielectric constant of 3.3 and a reasonably high breakdown strength (570 V/μ, 14 kV/mil.), but has a relatively high dissipation factor (˜10−2 at 1 kHz), which increases with temperature and frequency. PET is also limited by a maximum operating temperature of 125° C.
Thus, there is a current, pressing need for ultra-high energy density polymer dielectrics with a high dielectric strength, ideally, >650 V/μ or 16 kV/mil., with an acceptably low dielectric loss, dielectric constants in the 3.0-5.0 range and with preferred service temperatures exceeding 200° C. This means that they should have high glass transition temperatures (preferably 250° C. and above) and thermal and thermooxidative stabilities >400° C. High glass transition temperatures in polymer dielectric films can help delay the initiation of electromechanical breakdown under an applied electric field since the electrical breakdown characteristics of polymers in the temperature region near their softening points are similar to the changes in the film mechanical properties.
A rational choice for the design and evaluation of a polymer dielectric has to take into account the figures of merit provided by already existing materials paradigms; one such model is the polycrystalline CVD (chemical vapor-deposited) diamond film, known to be an exemplary high temperature capacitor. It has a relatively high dielectric constant (5.68), has the potential to store energy density of ˜10 J/cc at a breakdown strength of 16 kV/mil., and an exceptionally low dissipation factor (<10−5 at 1 kHz). Major drawbacks of the CVD diamond are costs, difficulty of large area synthesis and undesirable roughness of the growth surface.
Based on the favorable analogy of diamond, high temperature polymer dielectrics incorporating diamond-like hydrocarbon subunits (or diamondoids) in the polymer backbone were developed and evaluated as capacitor dielectrics for high voltage applications. The diamondoids are essentially hydrogen-terminated diamond fragments. This rationale is validated by the fact that large HOMO (Highest Occupied Molecular Orbital)-LUMO (Lowest Unoccupied Molecular Orbital) gaps in diamondoids are considered as molecular counterparts of a large fundamental band gap in diamond, which is responsible for its optical transparency in the visible region as well as its electrically insulating properties.